Ehrlichia ewingii, a tick-transmitted rickettsia previously known only as a canine pathogen, is the most recently recognized human granulocytic ehrlichiosis agent. Granulocyte-tropic Ehrlichia was first reported by Dr. S. A. Ewing in 1971 in a dog from Arkansas and was thought to be a granulocytic variant of Ehrlichia canis. Granulocyte-tropic Ehrlichia was recognized as a separate species in 1992, based on 16S rRNA gene sequence comparison and named as Ehrlichia ewingii in honor of Dr. S. A. Ewing. Since then, canine infection with E. ewingii has been reported in several states in the U.S. and recently from Africa. Clinical signs in dogs infected with E. ewingii are fever, lethargy, anorexia, lameness, and polyarthritis, accompanied with mild thromobocytopenia and mild anemia. In 1999, human infection with E. ewingii was documented. Since 1996, retrospectively, approximately 10 confirmed cases of human granulocytic ehrlichiosis caused by E. ewingii infection have been identified in Missouri and Oklahoma.
Diagnosis of E. ewingii infections has proven difficult. E. ewingii has yet to be cultivated, and there is no serologic test available to diagnose E. ewingii infection. Clinical signs of patients infected with E. ewingii, such as fever, headache, myalgia, leukopenia, and thrombocytopenia are similar to those of human monocytic ehrlichiosis caused by E. chaffeensis and human granulocytic anaplasmosis caused by A. phagocytophilum. Hence, clinical features alone cannot distinguish these causative agents. Further complicating the diagnosis of ehrlichiosis infections, E. ewingii and E. chaffeensis also share the same vector tick species and animal reservoirs. Experimentally, the Lone star tick (Amblyomma americanum) has been shown to be a competent vector, although bacterial DNA has been detected in other species of ticks. White-tailed deer (Odocoileus virginianus) is considered to be an important reservoir for E. ewingii and dogs are also possible reservoirs. Consequently, E. ewingii and E. chaffeensis have similar seasonal and geographic distributions. While bacteria have been seen on blood smears from infected animals and humans, and detected by PCR in the blood and tick specimens, to date E. ewingii remains uncultivable and a stable laboratory isolate is not available. PCR tests based on the E. ewingii-specific partial sequence of a 16S rRNA gene and a partial p28-19 sequence have been reported (Gusa, A. A., et al. 2001. J Clin Microbiol 39:3871-3876). Yet, sensitivities and specificities of E. ewingii PCR tests in clinical specimens are unknown, as there are no other definitive tests with which to compare. The microscopic observation of morulae in Romanovsky dye-stained peripheral blood granulocytes provides definitive proof of ehrlichial infection. Unfortunately, this test cannot be used as a single diagnostic test for E. ewingii infection because it cannot distinguish E. ewingii morulae from other granulocytic agents, such as A. phagocytophilum. Furthermore, negative results from Romanovsky dye-staining cannot rule out E. ewingii infection, owing to high false-negative rates caused by sample conditions and the low sensitivity of the assay. These setbacks in prior diagnostic testing necessitate an additional test to properly identify E. ewingii infection.